The present invention relates to an optical recording medium capable of storing and retrieving information by light exposure. More specifically, the invention relates to an optical recording disc comprising recording layers that have optical properties which change when the layers are exposed to a focused laser beam.
Optical discs are widely used in industrial and consumer recording systems such as video recorders, digital audio recorders and document digital recorders. A signal is recorded on the optical disc by applying a focused laser beam so that bits of information are formed in the optical disc. The optical disc permits direct read after write and random access to the recorded information.
Optical recording discs have been proposed which comprise a transparent substrate and thin layers of at least two materials supported on the substrate, wherein an energy beam, such as a focused laser beam, is directed to the thin layers to cause structural changes. When the optical disc is irradiated with the energy beam, the materials in the recording layers interchange or are mixed, so that recorded spots are formed in the thin layers due to the localized melting by the energy beam. The mixing of the two layers causes changes in the optical properties of the layers. In a subsequent reading operation, the spots with altered optical properties can be detected as the recorded spots.
An optical disc which discloses the concept of localized melting of the thin layers is reported by K. Y. Ahn in "Laser Writing On Metal-Silicon Bilayers For Optical Storage. I. Optical Properties," J. APPL. PHYS. P. 3777, Vol. 53, May 1982. Another example of an optical disc is disclosed in U.S. Pat. No. 4,477,819. This optical disc is based on the two layer structure as described above, wherein one of the first of the two layers comprises a metal and the other of the two layers comprises a metal or a semiconductor.
A serious disadvantage associated with optical discs with the two layer structure mentioned above is the accidental and unwanted interdiffusion of the two layers. As noted above, the recorded spots are formed when a laser beam causes the two layers to melt and mix thus changing the optical properties in a specific area. The two layers, however, will frequently mix at lower temperatures without exposure to the energy beam. Sunlight, or even room temperature heat can cause some unwanted mixing of the two layers. This mixing will result in a shorter lifetime of the disc and a low signal to noise ratio.